Flood inquiry relives 'unimaginable' storms

ENGINEERS operating south-eastern Queensland's biggest dam before the Brisbane flood based decisions on whether to release water on there being no further rain, an inquiry has heard.

As the first day of hearings into the state's deadly floods focused on dam management shortfalls, the Premier, Anna Bligh, was forced to hose down suggestions the government was hiding documents from the inquiry.

Yesterday the $15 million inquiry looked at the role of Wivenhoe Dam, which has a dual purpose of storing drinking water and minimising the impact of floods.

Under scrutiny ... the dam operator has faced criticism over water releases in the day before Brisbane flood. Photo: Dean Saffron

Its owner, Seqwater, has faced criticism over dam releases in the days before the Brisbane flood, amid claims that it did not let out water soon enough and was then forced to ramp up releases.

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A senior Wivenhoe engineer, Rob Ayre, was questioned about why his team did not rely heavily on rainfall forecasts when managing water releases into the Brisbane River before the city's flood peak on January 13.

Mr Ayre said rainfall predictions were volatile and he believed it was best to rely on a no-rainfall scenario.

"It's been proven to be the most reliable in the past," he said.

Mr Ayre was one of four engineers who decided on the timing and scale of hotly debated water releases into the Brisbane River based on the government-approved dam manual.

Engineers had "some communication issues" while trying to contact agencies, including Brisbane City Council, a few days before the Brisbane River peak, Mr Ayre said.

Earlier, the minister in charge of dams at the time, Stephen Robertson, told the inquiry he could not hand over certain documents because they were covered by parliamentary privilege. He insisted he was not being "deliberately obstructive" by refusing to release his parliamentary briefing notes regarding the Wivenhoe Dam supply level, saying Parliament would first have to approve a motion.

"Frankly, if it was up to me I would [hand over the documents]," Mr Robertson said during a heated exchange with counsel assisting the inquiry, Peter Callaghan.

"I can understand your frustration but I can only restate … it is not within my ability to waive parliamentary privilege."

Ms Bligh, who previously said the inquiry should leave no stone unturned, said the government would support moves to make the documents available.

"We'll certainly be moving in the Parliament to make that happen as soon as possible," she said.

The inquiry was told the government shelved plans to lower the storage level late last year because the proposed 5 per cent reduction would be "pointless" in avoiding floods.

Wivenhoe Dam can hold a total of 2.6 million megalitres but is deemed to be "full" of drinking water when it reaches 1.15 million megalitres, with the remaining space used as a flood mitigation buffer.

The inquiry, before Supreme Court justice Catherine Holmes, will also look at disaster preparedness, the emergency response, the performance of insurance companies, and development planning.